1. Field of the Invention
The invention relates to measuring inter-frequencies in a mobile telephone system employing frequency division duplex (FFD) and especially to optimising said measurements with respect to the total output in a system employing code division multiple access (CDMA).
2. Brief Description of Related Developments
Third-generation mobile telephone systems called UMTS (Universal Mobile Telephone System) and IMT-2000 (International Mobile Telephone System), for instance, will use wideband code division multiple access technology, i.e. WCDMA technology, on the radio path. In a WCDMA system, all mobile stations in a cell use the same frequency between each other on the transmission link from the mobile station to the base station and correspondingly, the same frequency between each other on the transmission link from the base station to the mobile station. A WCDMA system can in mobile telephone systems be implemented either as frequency division duplex (FDD) or time division duplex (TDD).
In an FDD-type WCDMA system, the uplink direction (from the mobile station to the base station) and the downlink direction (from the base station to the mobile station) transmissions are independent of each other. Thus, the base stations need not be synchronized with respect to each other, either. It is, however, typical of CDMA systems that a downlink transmission is performed simultaneously from several base stations to one mobile station, which transmission the receiver of the mobile station is arranged to receive. This arrangement is called a soft handover, and to control it, the mobile station must perform various parameter measurements for both uplink and downlink connections. Corresponding measurements are also used in updating the location of a mobile station and in handovers between WCDMA and GSM systems.
The receiver of a mobile station is typically arranged to receive only one frequency at a time, which means that one set of receiving means is enough for the mobile station and there is no need to design antenna diversity to them, which is advantageous both in view of cost and making the structure of the mobile station simple. The mobile station can also be designed to comprise several receiving means (dual receiver), which usually include antenna diversity. This type of mobile station is, however, more expensive and complex to implement.
Thus, the parameter measurements described above can be performed in a typical one-receiver mobile station only when there is no transmission. This also applies to dual-receiver mobile stations when one set of transmission/reception means transmits on almost the same frequency as a second set of transmission/reception means performs measurements. In an FDD-type WCDMA system, the transmission is interrupted for a while by generating in a frame a gap during which transmission is interrupted. This is done by using what is known as compressed mode or slotted mode in which information normally transmitted in a 10-ms frame is transmitted in a shorter time. Since the same information is transmitted in a shorter time, a gap remains in the frame, during which measurements of the parameters described above can then be performed. Depending on the measurement situation and the transmitter properties, compressed mode is only used in uplink or downlink transmissions, or a combined uplink/downlink compressed mode can also be used.
In CDMA systems, all mobile stations connected to a certain cell typically use the same frequency bands in uplink and downlink transmissions, which means that the transmissions of various mobile stations and base stations cause interference to each other. In addition, due to the signal propagation mechanism, signals transmitted from a mobile station close to a base station arrive at the base station stronger than those transmitted with the same power but further away from the base station, i.e. what is known as a near-far effect takes place. To maximize the capacity of a CDMA system in relation to the radio interface, it is essential that signals arriving at the base station have substantially the same average power, i.e. signal-to-interference ratio (SIR). Owing to this, CDMA systems are characterized by a fast but complex transmission power control method.
In the compressed mode described above, a certain amount of data to be transmitted is compressed to be transmitted in a shorter time, in which case the transmission power must be momentarily increased to maintain a constant signal-to-interference ratio. Increasing the transmission power then also causes interference to the transmissions of mobile stations in the same cell, and they, too, need to increase their transmission power to compensate for the interference.
A problem with the above arrangement is that the present WCDMA system does not define where in a time division frame compressed mode is used, in other words, where in the frame a gap is generated for measuring parameters. Thus, the gaps may randomly fall anywhere in the frame. If in several mobile stations, the gap falls into substantially the same place, the compressed places in which transmission power has been increased, also overlap at least partly. The total interference in the system then increases and the average transmission power of the mobile stations must be increased. Further, increasing the transmission power may cause an uncontrolled state in which all mobile stations increase their transmission power to its maximum, whereby the capacity limit of the system is reached and the quality of the transmissions decreases. Because the base station, too, typically transmits at the same frequency as the mobile stations, the downlink direction transmission power must also be increased.
In one embodiment, the present invention is directed to a method for defining measurement gaps in a wireless telecommunications system comprising one base station and several wireless terminals, the method comprising: defining for the terminals in said telecommunications system measurement patterns which define locations of gaps used for measurements in a time-slot frame, and sending the measurement patterns to the corresponding terminals through the base station. The method is characterized in that various delays are defined for the measurement patterns of said terminals so that the gaps of different terminals are in substantially different locations in the time-slot frame.
The invention also relates to a wireless telecommunications system comprising a fixed network, at least one base station and several wireless terminals and means for defining measurement patterns for terminals, which measurement patterns define locations of gaps used in measurements in a time-slot frame and which base station comprises a transmitter for transmitting the measurement patterns to the corresponding terminals. The telecommunications system is characterized in that said means for defining measurement patterns are arranged to define various delays for said terminal measurement patterns so that the gaps of different terminals are in substantially different locations in the time-slot frame.
The invention further relates to a terminal in a wireless telecommunications system, which terminal comprises a receiver for receiving measurement pattern definitions defined by the telecommunications system and processing means for arranging gaps in a time-slot frame according to the measurement pattern definitions, and which is characterized in that said processing means are also arranged to set for the measurement pattern a delay according to the measurement pattern definitions.
The invention also relates to a base station in a wireless telecommunications system, to which base station means are operationally connected for defining measurement patterns for terminals, which measurement patterns define locations of gaps used in measurements in the time-slot frame and which base station comprises a transmitter for transmitting the measurement patterns to the terminals. The base station is characterized in that said means for defining measurement patterns operationally connected to said base station are arranged to define various delays for said terminal measurement patterns so that the gaps of different terminals are in substantially different locations in the time-slot frame.
The invention is based on the idea that to make measurements, especially those performed in compressed mode, non-simultaneous, at least partly different delays are set for the measurement patterns defining the measurement gaps of each mobile station, whereby time-slots also transmitted in compressed mode at a higher data rate are distributed more evenly to various mobile stations in relation to the time-slot frame.
The method and system of the invention provides the advantage that by optimizing the distribution of the measurement gaps between various mobile stations, the interference caused by mobile stations to each other at a higher transmission power is reduced. This provides the further advantage that the average transmission power of the system remains low, thus improving the capacity of the system.